This application relates to a process for reactivating catalysts having noble metals, such as platinum, supported on microporous crystalline oxide materials containing aluminum and phosphorus.
Noble metals are known to have catalytic activity in converting organic compounds. For example, such metals may be used in the hydrogenation and dehydrogenation of hydrocarbons. It will be noted that noble metals, such as platinum, are extremely expensive and rare. Accordingly, when these materials are used as catalysts they are generally uniformly distributed on a suitable support material. In this regard, the noble metal should be as finely dispersed as possible in order to provide a maximum surface area for contact with reactant molecules.
A number of materials have been used to support noble metal catalysts. These materials may be either essentially catalytically inactive or also possess catalytic properties which function in an additive or synergistic fashion with the catalytic properties of the noble metal. An example of an essentially catalytically inactive support material is gamma-alumina. An example of a catalytically active support material is an acidic aluminosilicate zeolite. Such zeolites have acid catalytic activity which may be used for a variety of organic compound conversions such as the cracking of hydrocarbons. Accordingly, a noble metal suitable supported on an appropriate zeolite may provide an excellent hydrocracking catalyst, wherein acid sites on the zeolite promote the cracking of hydrocarbons and the noble metal, in close proximity to these acid sites, promotes the hydrogenation of the cracked products.
Zeolites, in addition to providing further catalytic activity, may provide a means for achieving shape selectivity for reactions catalyzed by noble metals. More particularly, such zeolites may have a porous network of channels which are large enough to freely permit entry of molecules of small diameter, such as straight chain paraffins, while tending to restrict molecules of larger diameter, such as aromatics or branched aliphatics. When noble metals are incorporated in the channels of such a zeolite, an isomerization catalyst for dewaxing hydrocarbons may be provided, wherein straight chain wax molecules are preferentially admitted into the channels and are converted to branched hydrocarbons under the influence of noble metal catalytic species. Note U.S. Pat. No. 4,419,220, the entire disclosure of which is expressly incorporated herein by reference.
When solid support or catalytic materials are exposed to hydrocarbons at elevated temperatures for prolonged periods of time, a dense hydrocarbonaceous deposit (e.g., coke) can tend to form on the solid materials. This coking process can deactrivate catalytic materials. One way of removing coke from deactivated catalysts is to oxidize (e.g., burn) the hydrocarbonaceous deposits by exposing the catalyst to a source of oxygen (e.g., air) at an elevated temperature. However, the severe conditions encountered in such oxidations can have a detrimental effect on certain supported noble metal catalysts. More particularly, as pointed out in U.S. Pat. No. 4,657,874, the entire disclosure of which is expressly incorporated herein by reference, when highly siliceous noble metal-containing zeolites are subjected to coke-burnoff, the noble metal thereof agglomerates, thereby substantially reducing the surface area of the noble metal. Note particularly, Example 5 of U.S. Pat. No. 4,657,874. The agglomerated noble metal on the zeolite can be redispersed by certain chemical treatments, but, as pointed out in this Example 5, this redispersion falls short of achieving the original level of high dispersion of noble metal before agglomeration.
The nature of the support material can have a profound effect on the manner in which noble metals can be distributed thereon under various conditions. As mentioned hereinabove, noble metals can be very finely distributed on highly siliceous zeolites. However, when subjected to the conditions of coke burn-off, this distribution is disturbed and agglomerates of noble metals form. The noble metal in these agglomerates can be only partially redistributed on the surface of the highly siliceous zeolites. In contrast to the surface of highly siliceous zeolites, the surface of gamma-alumina tends to more tenaceously hold noble metals. More particularly, noble metal supported on gamma-alumina will tend to agglomerate to a much less extend when such materials are subjected to the conditions of coke burn-off. The ability of a support material to inhibit the agglomeration of noble metals thereon and to promote the redispersion of noble metals thereon is apparently a function of the surface chemistry of the support material, e.g., in terms of the charge and charge distribution thereon.
Examples of materials which have an entirely different surface chemistry than highly siliceous zeolites are three-dimensional microporous crystal framework structures consisting essentially of corner-sharing oxide tetrahedra of alumina and phosphorus. An example of such a material is termed an aluminophosphate in U.S. Pat. Nos. 4,310,440 and 4,385,994, the entire disclosures of which are expressly incorporated herein by reference. The aluminum/phosphorus atomic ratio of these aluminophosphate materials is about unity, the framework positive charge on phosphorus being balanced by corresponding negative charge on aluminum: EQU AlPO.sub.4 =(AlO.sup.-.sub.2) (PO.sup.+.sub.2).
Other materials which consist essentially of corner-sharing oxide tetrahedra of aluminum and phosphorus are those wherein minor portions of such tetrahedra are replaced by oxide tetrahedra of other elements. An example of such a material is a silicophosphoaluminate (i.e. SAPO) as described in U.S. Pat. No. 4,440,871, the entire disclosure of which is expressly incorporated herein by reference. Other examples of such materials containing tetrahedral oxides of aluminum and phosphorus are termed ELAPSOs, MeAPOs, FeAPOs, TiAPOs and FCAPOs, which are described in PCT International Publication No. WO 86/03139, the entire disclosure of which is also expressly incorporated herein by reference.
Prior to the time of the present invention, it was uncertain whether or not noble metals would tend to agglomerate after being dispersed on crystalline aluminum and phosphorus oxide containing materials such as aluminophosphates, SAPOs and ELAPSOs. More particularly, when subjected to the conditions of coke burn-off, it was uncertain whether such materials would behave as noble metals supported on highly siliceous zeolites, whereby the noble metals would become extensively agglomerated, or whether such materials would behave as noble metals supported on gamma-alumina, whereby the noble metals would resist extensive agglomeration and remain essentially dispersed. Surprisingly, it was discovered that noble metals became extensively agglomerated on these aluminum/phosphorus oxide supports.
Once the noble metal agglomerates were formed, it was further uncertain whether the noble metal agglomerates could be redispersed. There was certainly no reason to expect that noble metal agglomerates could be redispersed to a greater extent than observed for such redispersion on highly siliceous zeolites. However, it was surprisingly discovered that these agglomerates were redispersed on the aluminum/phosphorus oxides to a much greater extent than observed for the highly siliceous zeolite supports. Quite unexpectedly, the original extensive level of noble metal dispersion was observed after the redispersion treatment of the noble metal supported on the aluminum/phosphorus oxides.